For a million years you are being trained to reason about the physical world as perceived by your sensors. You evolved to search for patterns and assume animal agency by default (simply because the cost of the mistake is lower with this assumption). Then came computer programs… they are invisible for your sensors, they do not follow any patterns, they can make computers appear animate, they can disguise as a reasonable actor, or fool your senses otherwise. And on top of it all they do not obey the laws of physic, the laws that your brain perceive as unbreakable for any agent in the visible world. This is a disaster for your neolithic brain.
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I have a strong conviction that 99% of «security experts» do not know the definition of the entropy. This conviction does certainly seem wildly deranged for you, unless you know the definition in question. So, let's begin with the definition, by the book.
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Previously (in "What Makes Your Password Yours" and "Auth vs ID") We have established that your exclusive and complete control over your password makes the password belong to you, and this is the one and only characteristic property of a valid password. Now, we are to scrutinize the second member of the 2-Factor formula.
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It turned out that my "Authentication vs Identification" article was not sufficiently conclusive in the sense that some hardcore biometrics fans still nurture a non-trivial and well justified objection. So I need to address and destroy it, in order to close the topic. My opponents' argument is:

Your analysis narrows the both sides of the problem to a knowledge/ownership claim. Even if you are right, the conclusion is only applicable to the authentication by means of a knowledge token, whereas all the rest relations between the user and the token (suitable for authentication purposes) are set aside. There is one particularly important relation (the one fundamental for the entire biometrics field): «the user is» or other way around «the token is a part of the user» — this relation implies inalienability which makes the token safe for authentication purposes.

It is true. Completely true. It is undeniably true! In the physical realm.
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I keep telling you time and again «there is no silver bullet in Information Security», despite the vendors blatant attempts to claim otherwise.

You cannot buy a box that solves your problems at once. Every tool needs a skilled person to be mastered by. And still we look forward with hope for the best and the victory of reason, while, trying to guess the shape of things to come. There are emerging technologies, deep learning, AIs and stuff that certainly will change everything. Someday. And there are promising startups that already started doing it. But is it really the most important change we expect — right now?
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Once again I have to return to the topic of strict antagonism between the authentication and the identification, meaning these very processes and the tokens involved as well. Before I indulge into boring you with tedious decomposition of entities you used to perceive as atomic, I present you a synthetic illustration of the difference in question. A bad guy tries to get a false-negative outcome of identification, and a false-positive outcome of authentication. This is not explanatory, yet very indicative, I hope it gives you an idea of the magnitude of the difference, and we are going to dig into this now.
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Simple questions are usually the most difficult ones to answer. And the most important among them are traditionally labeled stupid and dismissed. The modern days InfoSec is based upon unanswered questions. The lack of theoretical basis allows InfoSec gurus to produce teachings and «best practices» without a limit.

Today I want to address two very basic questions about passwords:

What are characteristic properties of a password? and what makes your password yours?

By answering these questions you achieve understanding of the utter malevolence of the password abandonment movements, that are so frighteningly popular today. There is a particularly dangerous movement to replace passwords with bio-metric attributes that can reliably identify your body (e.g. voice, fingerprints, and such). Although these attributes are successfully used in forensic practice for centuries, it does not make them good authentication tokens. Why? Because your password's job is NOT to identify your body.

I hear you screaming: «WHAT?!?!?!» That means you are ready to investigate what IS a password, what is its job, and what properties do you want your password to possess.
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I suggested in the article to use passphrases instead of «traditional» passwords, for multiple reasons, including: sheer strength, memorability, and conforming to idiotic password creation policies without actually following detrimental recommendations of the policy authors.

This recommendation gives rise to a reasonable doubt: «what if syntactically correct phrases are as weak as dictionary words in comparison to a random string of symbols?''. Indeed, syntax itself should weaken a passphrase, as it provides some „predictability'' to the phrase. I want to address this problem, by comparing syntactically correct passphrases to random collections of words (which we all consider sufficiently strong… hopefully).
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It is also worth noticing that Shannon's entropy is entirely irrelevant to the password strength problem. Entropy is based on the ASSUMPTION of possible outcomes. This set is well defined in the context of measuring a memory size, and not defined at all in the context of password creation/guessing.

In contrast to my work, the «password strength» discourse is extraordinarily rich on bullshit. Take a look at this masterpiece. This is a great example of how to write some 35K of text without answering the question, and even without understanding the subject. Let me quote the key paragraph:

Password strength is a measure of the effectiveness of a password in resisting guessing and brute-force attacks. In its usual form, it estimates how many trials an attacker who does not have direct access to the password would need, on average, to guess it correctly. The strength of a password is a function of length, complexity, and unpredictability.

This is one of the best non-answers I have ever seen, and a very succinct one too. No wonder it originates from the government officials! It obscures the matter it addresses in almost every word — «effectiveness», «resisting», «complexity», «unpredictability» — what are they? So, essentially the quoted paragraph reads:

Password strength is a function of something unknown to us.

It is time for us to do some trivial maths and terminate the «password strength» nonsense.